The long-term goal of these experiments is to understand the role of the vestibular system in maintaining stance and balance. The ability to maintain stability during stance is fundamental for the successful performance of many functional motor tasks, and yet little is known about the mechanisms that the CNS uses to solve the complex control problems of stance. Dizziness is one of the most common complaints faced by clinicians, and symptoms of dizziness are often accompanied by problems with balance. A significant proportion of these problems of instability could be related to pathology, injury, or degeneration in the vestibular system. Therefore, it is vital to understand how vestibular inputs are used by the motor system to maintain stance and stability. Although vestibular lesions are known to be associated with postural instability, the role of the vestibular system in triggering and controlling stabilizing reactions is not well-understood. It is commonly accepted that vestibulospinal and cervicospinal reflexes underlie the functional stabilizing reactions in the intact animal, but this theory has never been adequately tested. Moreover, there are considerable gaps in our knowledge of both the specific effects of these reflexes on the limb musculature, and of the stabilizing reactions used during stance in the awake, freely-standing animal. The experiments in this proposal are designed to fill these gaps, and to compare the reflex patterns with the functional responses of the intact animal, in order to test the hypothesis that the vestibulospinal and cervicospinal reflexes constitute functional mechanisms for stance. The rationale is to determine whether the relevant reflexes and the stabilizing reactions both activate the same muscle patterns. The Specific Aims of this proposal are: 1. to characterize the stabilizing reactions that accompany voluntary head movements and static head postures in intact and labyrinthectomized cats, 2. to determine the static and dynamic responses of cats to tilting of their support surface, and 3. to characterize vestibulospinal and cervicospinal reflexes in the decerebrate cat whose neck is held vertically. If the muscle activation patterns of the awake, standing cat (Specific Aims 1 and 2) are similar to the relevant reflex patterns in the decerebrate cat (Specific Aim 3), this would provide good evidence that the vestibulospinal and cervicospinal reflexes are functional mechanisms that are used for maintaining stance and balance in the intact animal. These studies will improve our understanding of the functional postural mechanisms in the awake cat, and will also provide insight about the source of balance problems and cervical complaints in patients with vestibular deficits.